Process for the manufacture of two-wheeled-vehicle tires

ABSTRACT

A process for the manufacture of tire for high speed two-wheeled vehicles comprising making a cylindrical sleeve, attaching annular bead wires to the axial ends of said sleeve and shaping the sleeve into a toroidal configuration. A belt structure made of coils of cords is applied to the crown of said toroidal configuration. The belt is reinforced with HE cords which can be deformed to enable the tire carcass to be so largely expanded into the mold to achieve completely molding of a tread band.

This application is a Continuation of application Ser. No. 07/714,871,filed Jun. 13, 1991, now abandoned.

BACKGROUND OF THE INVENTION

The present invention is concerned with a process for the manufacture ofmotor-driven vehicle tires, in particular for two-wheeled vehicles andmore particularly for motorcycles, and the tires made by the process.

These tires have a particular structure and shape so that they canwithstand use conditions completely different from those in which normaltires for four-wheel vehicles are used; it is in fact sufficient toremember that when curvilinear paths have to be taken it is not thetravel direction of the tire which is altered but the vehicle position,which is inclined sideways towards the road surface during a turn.

For the above reason two-wheeled-vehicle tires must be capable ofrunning with camber angles of very high value, in the order of 50° ormore, against a normal value of 3° to 4° in four-wheeled-vehicle tires,the camber angle being continuously and uniformly altered: consequentlythese tires have a cross section which is very rounded, that is providedwith a strong convexity in the crown, unlike other tires in which thetransverse profile of the tread is almost flat.

This strong convexity in the crown involves problems during the tiremolding step bringing about drawbacks and deterioration of quality inuse.

THE PRIOR ART

It is known that a tire of the above type comprises a carcass oftoroidal form, a tread band disposed on the crown of said carcass and anannular reinforcement structure, also known as belt, fitted between thecarcass and the tread band.

In particular, in carcasses of the radial type, that is those havingreinforcement cords lying in planes containing the rotational axis ofthe tire, the belt preferably comprises one or more layers of cords madeof an appropriate reinforcement material, at least resistant to tensilestress, disposed in mutual side by side relation and all parallel to oneanother and oriented in a substantially circumferential direction.

This annular reinforcement structure must be circumferentiallyinextensible and be submitted to a tensile preloading when the tire isdeflated, so that it can perform all the required functions when thetire is in use.

It is also known that a process for the manufacture of such radial tiressubstantially comprises the steps of disposing the carcass in the formof a cylindrical sleeve on an appropriate manufacturing drum; givingsaid carcass a toroidal form on the selfsame drum or on a differentmanufacturing drum; applying an annular reinforcement structure and thetread band to the crown of the toroidal carcass thus shaped; putting thecarcass thus arranged within a vulcanization mold of the type commonlyreferred to as "centripetal"which, in register with the tread, isprovided with a ring of sectors radially movable inwardly and outwardly;expanding the carcass for the purpose of putting said belt in apreloaded condition and simultaneously closing the mold by a centripetalradial displacement of the sectors, so that a raised pattern formed bysaid sectors may penetrate into the tread band while carrying out themolding thereof.

The expansion of the carcass within the mold is usually known as"pulling" and corresponds to an increase of about 1.5% in thecircumferential size of the tire. This process, in tires of the typehaving a high transverse convexity to which the present inventionrefers, gives rise to some drawbacks which are particularly troublesomein tires in which the tread pattern has very deep hollows or grooves.

In fact, as previously stated, the belt must be an almost inextensiblecircumferential structure, which means that the maximum permissiblepulling cannot exceed the above stated value corresponding to a similarpercent increase in the diameter thereof.

If the maximum depth of the hollows in the tread pattern is higher thanthis value it is apparent that the overall pattern molding can only beachieved by making the ribs present on the mold sectors penetrate intothe thickness of the tread band and therefore the deeper the ribpenetration is, the greater the portion of hollow formed in this manner.

It is however to be noted that while the expansion of the carcass issubstantially radial at any point, and therefore the hollow portionsformed as a result of the movement of the tread band against the sectorribs have a certain radial height, on the contrary the movement of eachsector against the tread band is radial only at its axis of symmetry,whereas all other points move in a direction parallel to said axis andtherefore not at right angles to the profile of the tire carcass,thereby molding hollows that are offset and displaced with respect tothe desired pattern.

The phenomenon is increasingly more serious at the sector ribs which aremore spaced apart from the axis of symmetry and for tires having astronger convexity, particularly in the transverse direction.

In order to solve this problem the expansibility of the belt should beincreased so as to increase the depth percent of the hollow formed bythe movement of the tread band against the mold sectors (at the most asfar as the value of 100%), but this would require a belt having a veryexpansible circumference which is contrary to the requirements of thetire in use.

The manufacture of a belt which is extensible only during the moldingstep by the use of the presently adopted cords made of high-resistantmaterials such as Kevlar (registered trademark available from Du Pont)or steel, but laid down loosely, that is in an undulated manner, or incompression, is not feasible due to the difficulties arising inconnection with the process; actually the number of problems created bysuch a working process is greater than that of the problems thus solved.

Neither does the use of very extensible and heat-shrinkable textilematerials such as nylon enable the problem to be solved, since thesematerials are unable to withstand stresses arising during the use of thetire without undergoing important deformations, in particular due to theeffects of centrifugal force connected with high speed of the vehicle,which can even reach values in the order of 300 kilometers per hour.

SUMMARY OF THE INVENTION

The applicant has now devised a new process for the manufacture of tiresof the type discussed above, and in particular a process for carcassshaping and belt manufacture enabling all the above problems to besolved and still other problems that will be disclosed hereinafter, sothat the object of the present invention is a process for themanufacture of tires For motor-cycles capable of offering highperformances, and the tires thus obtained that are in particular freefrom structural and molding faults that could adversely affect theirbehavior in use.

One aspect of the invention is to provide a process for the manufactureof tires for vehicle wheels comprising the following steps:

manufacturing a tire carcass in the form of a cylindrical sleeve, byproducing a tubular element of rubberized fabric reinforced with cordsdisposed substantially parallel to the axis of said sleeve and foldingback the ends of the cords of said tubular element about one of a pairof annular metal bead cores which are circumferentially inextensible,said bead cores lying in planes perpendicular to the sleeve axis;

shaping said tubular element into a torus by radially expanding thesleeve wall and moving said bead cores close to each other in an axialdirection, so that a first toroidal configuration is formed;

assembling a belt structure to the crown of said torically-shapedcarcass, which belt structure comprises cords oriented in asubstantially circumferential direction disposed in mutual side by sideparallel relation, and superposing a tread band on said belt at aradially outer position thereof;

introducing the assembled tire into a vulcanization mold;

closing said mold and expanding the carcass against the mold surface, sothat it reaches its final toroidal configuration, so as to achieve themolding of the tread pattern into the tread band in particular; and

vulcanizing said carcass, the process being characterized by the factof:

making said belt using cords of a high-elongation type exhibiting, intheir load-elongation diagram, a curvilinear portion mutually connectingtwo substantially rectilinear lengths to each other, which lengths areof different inclination relative to the axes of said diagram, and

moving said annular bead cores coaxially close to each other during thetoroidal shaping of said tubular element until a mutual axial distanceis reached which corresponds to an intermediate toroidal configurationof smaller diameter than that of said first toroidal conformation, andexhibits a deviation of predetermined value from said final toroidalconformation, the value of the center axial distance between saidannular bead cores being given by the load-elongation diagram of saidbelt cords, so that the expansion of said carcass in said mold to thefinal toroidal configuration, puts said belt cords in a tensilecondition (preloading) corresponding to a point of said diagram locatedwithin said curvilinear portion.

Preferably said point will be located close to the end of saidcurvilinear portion corresponding to the highest elongation value.

In a very convenient manner, said cords are metal cords oriented in thesame way and exhibiting an ultimate elongation included between 4% and8% and which have been previously rubberized.

Conveniently said belt structure can be made by wrapping one or moreturns of one or more strips of sufficient width around said carcass,which strips are formed with rubberized fabric reinforced with saidhigh-elongation cords, overlapping the ends of said strip or strips overa portion in the order of 20 mm to 50 mm, or wrapping a singlehigh-elongation cord continuously and repeatedly around said carcassgoing on in an axial direction from one end of the carcass to the other.

A different way for making said belt comprises helicoidally wrapping orcoiling a tape of rubberized fabric of two or more (usually ten at themost) high-elongation cords on said carcass, proceeding in an axialdirection from one end to the other along the toroidal profile of thecarcass. In addition, should the above coiling techniques be used, thehelical wrapping can also be carried out by simultaneously proceedingfrom the middle towards both ends of the carcass, or the other wayround.

In keeping with a further aspect of the invention, a tire fortwo-wheeled vehicles is provided which comprises a carcass of toroidalform having a strong transverse convexity, having a crown portion andtwo sidewalls terminating in beads for anchoring to a correspondingmounting rim, a tread band located on the crown of said carcass andhaving a convexity ratio in the range of 0.15 to 0.45, and acircumferentially inextensible belt structure interposed between saidcarcass and tread band, said annular reinforcement structure comprisinga plurality of cord coils disposed in axial side by side relation,extending from one end of said structure to the other according to anangle almost of zero value relative to the circumferential direction ofthe tire, characterized in that said reinforcement cords are of thehigh-elongation type having a load-elongation diagram with a curvilinearportion connecting two substantially rectilinear lengths to each other,said lengths having a different inclination relative to the axes of saiddiagram, said cords in a vulcanized but not inflated and not loaded tirebeing in a tensile condition (preloading) corresponding to a point ofsaid diagram located within said curvilinear portion and preferably inthe vicinity of the end thereof corresponding to the greatest elongationvalue.

In a preferred embodiment, said reinforcement cords are metal cordsoriented in the same way and having an ultimate elongation in the rangeof 4% to 8%.

The density of said cords in the above tire is preferable between 25 to150 wires/dm and said cords can also be unevenly distributed in an axialdirection, having concentrations and thinnings along said axialextension.

In addition, said high-elongation cords, if metallic, preferably consistof 1 to 4 strands, each strand in turn preferably consisting of 1 to 7elementary wires, of a diameter in the range of 0.10 mm to 0.25 mm andthe strands being helicoidally twisted together in the same way, theirstranding pitch being between 10 mm. and 200 mm.

Advantageously the tire of the invention shows a substantially balancedand uniform behavior, characterized by the fact that the differencebetween the values of the drift thrust measured on the tire duringrotation either clockwise or counterclockwise is less than 15% of thevalue of the maximum drift thrust.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be best understood from the followingdetailed description and the accompanying drawings, given hereinafter byway of non-limiting example, in which:

FIG. 1 is an axial cross section of the tire of the invention;

FIG. 2 is a qualitative and non-quantitative load-elongation diagram ofthe cords in accordance with the invention as compared with that of acord type widely used in the present tires;

FIG. 3 is a diagrammatic axial cross section showing the profile oftoroidal conformation of a tire carcass in accordance with the inventionin three different situations;

FIG. 4 is a plan view of a portion of the belt structure of the tireshown in FIG. 1, in accordance with a specific embodiment.

DESCRIPTION OF A PREFERRED EMBODIMENT

Illustrating the tire of the invention first and referring particularlyto FIG. 1, the tire comprises a carcass 1 formed with one or morerubberized fabric plies, having a toroidal conformation in right sectionin which the ends of the cords of the plies are folded back from theaxial inside to the axial outside in a known manner around annular metalbead cores 3, provided, at an outer radial position, with a filler 2 ofelastomeric material having appropriate rheometric features.

The crown of said carcass has a tread band 4 and an intermediatestructure 5, usually known as a breaker or a belt, interposed betweenthe tread band and the carcass.

The tire area comprising the bead core 3 and filler 2 constitutes thetire bead, for ensuring the anchoring of said tire to a correspondingmounting rim, not shown.

The area substantially included between the radially outer end of thefiller 2 and the belt 5 end is referred to as a tire sidewall andconstitutes the area of maximum flexibility, adapted to ensure comfortduring the vehicle ride.

On the other hand, in some very particular embodiments the radial lengthof the sidewall area can be greatly reduced and in effect the sidewallcan be eliminated when the radially outermost area of the filler 2 comesinto contact with the radially inner end portion of the tread.

As can be seen from the tire shown in FIG. 1, though only examined froma qualitative point of view, motor-cycle tires have a very encirclingtread, having a marked convexity in crown and, in effect, have a veryrounded shoulder or could be considered not to have a shoulder betweenthe sidewall and the tread band.

In particular this convexity is expressed by the value of the h/w ratio,where h represents the radial height of the tread relative to thereference line c--c passing through the radially inner ends of saidtread, and w represents the maximum width of said tread, as shown inFIG. 1. For the tire of this invention the h/w value is preferably inthe range of 0.15 to 0.45.

The exact defining of the tread ends, necessary for a reliable andobjective definition of the values of h and w, can be easily achieveddue to the fact that the tread edge is in register with the maximumwidth of the tire and often ends in a sharp edge distinguishing thebeginning of the sidewall and above all is adapted to enable the vehicledriver to appreciate when the maximum permissible camber angle isachieved, during a curvilinear ride path or on a curve in the road.

The radial height f or this edge or of the tread edge relative to a baseline (b--b) of the beads, taken as reference line, conveniently althoughnot necessarily is coincident with the ends of belt 5. The base lineb--b connects the bead seating area of the two tire beads.

Formed in the thickness of the tread band is a raised pattern consistingof a plurality of hollows or grooves 4a, 4b, 4c and lands (not shown)obtained in a known manner by a molding operation carried out on the rawcarcass in an appropriate mold.

The reasons for which in case of radial tires the mold must be of thetype provided with radially movable sectors, the so-called centripetaltype, have already been discussed.

For an appropriate behavior of the tire on the road, the direction ofthe grooves in relation to the thickness of the tread band must besubstantially perpendicular to the tread surface at that point. It willbe therefore recognized, and as can be seen from FIG. 1, that grooves ofthe same radial depth s actually have different developments into thetread, in a direction parallel to the equatorial plane m--m, dependingupon the lying position of the groove in the tread, this developments isequal to s for grooves located in the equatorial plane itself andproportionally decreases to a minimum value r for the grooves located atend (axial outer) positions.

On the contrary, when progressively moving axially from the middletowards the tread edges the width of said grooves in the axial directionincreases.

It is therefore apparent, also from FIG. 1, that a correct molding ofthe axial outer end grooves of the tread cannot be achieved by a moldportion (sector) which is moved as a unit in a radial direction parallelto the line m--m, although provided with ribs that exactly correspond tothe grooves 4b and 4c.

As regards the carcass, the reinforcement cords 11 (see FIG. 4) of thecorresponding plies are of any convenient known material, even ametallic but preferably a textile material, and in this case morespecifically of a material having a low modulus: among these materialsthe most appropriate for use are rayon, synthetic polyester fibers, andsynthetic aliphatic poliamide fibers, generally known as nylon.

For example, in the illustrated embodiment the cords are made of rayon,that is a synthetic fiber made from cellulose, extend axially from onebead to the other and are disposed at right angles to thecircumferential direction of the tire, shown by the line m--m of theequatorial plane (center line), that is inclined to said line by anangle substantially equal to 90°.

Considering now the belt structure 5, it is formed with a plurality ofcords 6 formed as coils disposed in axial side by side relation andextending from one axial end of the carcass crown portion to the other.

In a preferred embodiment of the invention said cords are metal cordsoriented in the same way, of the high-elongation type, generally knownas "HE cords" , each consisting of a certain number, 1 to 5 andpreferably 3 to 4, strands, each strand being comprised of a givennumber, 2 to 10 and preferably 4 to 7, of elementary wires having adiameter greater than 0.10 mm, preferably ranging between 0.12 and 0.25mm. The strand wires and the cord strands are helically twisted togetherin the same way, according to the same twisting pitch or also accordingto different pitches for the wires and the strands (so called Lang'slay).

Obviously materials other than metal could also be used, provided thatthe cords have the same type of load-elongation diagram as above statedand mechanical strength features adapted to withstand the forces actingon the tire.

In particular the cord used in the prototype tire herein specificallydescribed is a metal cord technically known as 3×7×0.12 HE, that is acord consisting of three strands, each of seven steel wires of 0.12 mmof diameter.

Due to the fact that the wire and strand twisting is carried out in thesame way, the finished cord is coiled and substantially acquires thefeatures of a spring and its particular load-elongation diagram shown inFIG. 2 by line HE depends on this fact.

In FIG. 2 there is a pair of reference axes perpendicular to each otherwith the load values C, that is the values of the tensile stressesapplied to the cord, reproduced on the vertical axis, whereas on thehorizontal axis the percent cord-elongation values L are reproduced.After submitting the cord to increasing tensile values, thecorresponding elongations are measured and then marked in the planedefined by said pair of axes at points corresponding to each pair ofapplied load/achieved elongation values until the breaking point of thecord is reached.

One can immediately see that said diagram includes one substantiallyrectilinear length (OE) of weak sloping (strong elongations at lowloads) followed by a still substantially rectilinear length (FZ) ofstrong sloping (small elongations at increasingly stronger loads), thetwo lengths being connected to each other by a curvilinear portion (EF)usually referred to as a "knee".

By way of example, one can ascertain that the center line of saidcurvilinear portion EF corresponds to an elongation value rangingbetween about 1.5% and 3%; thus the explanation of the diagram can beeasily understood.

The length of weak sloping corresponds to the elongation of the abovedescribed "spring"; in other words, during starting the elongation,first diagram portion, the helical coils of the cord extend, whichbrings about the wire straightening, as they exhibit a low tensilestrength.

Going on in pulling, at the end of the knee the wires have already beencompletely extended, though twisted upon themselves and therefore theyreact to the applied pulling force by effect of the mechanical featuresof the material and no more by effect of the geometrical configurationof the cord. The material used is steel and consequently the cordelongation becomes moderate in spite of the increase in the appliedforce.

On the contrary, the cords normally used exhibit a load-elongationdiagram as represented in FIG. 2 by line PA: in this case one can seethe almost constant behavior of the cord submitted to pulling stresses,also characterized by a lower sloping than in the cord HE; theimportance of this fact will be apparent in the following.

It will be now understood how the use of said cord has enabled theproblem of the invention to be generally solved. The cord is in factcoiled on the carcass shaped according to an intermediate toroidalconfiguration, still far from the final toroidal configuration that itwill take in the mold, under a rather weak tension in order not todeform the carcass structure.

In this manner the cord, with reference to its load-elongation diagram,is still in the vicinity of point O, in the starting length OE of weaksloping.

The belt is therefore substantially extensible at low values of theapplied load, so that the following carcass expansion in the mold(obtained with load C1) can conveniently take high values in order toenable the molding of the tread band, and in particular of the patterngrooves through the complete depth thereof, by the displacement of theband against the projections of the sectors in the already closed mold,thereby ensuring a strictly radial molding at all points, whereas thebelt cord or cords are brought to work to a point of the load-elongationdiagram included in said curvilinear portion and preferably close to theoutlet F from the knee and therefore already in a state of considerabletension (preloading) and in any case at the beginning of the length ofmaximum tensile strength.

The vulcanization of the tire makes this situation stable so that,during the use of the tire (strong increase of the permanent load(C2-C1) by effect of the inflation pressure, and strong variable actingloads (C3-C2) by effect of speed) the belt works in the cord diagramportion external to point X.

It will be immediately noted that in this portion of the diagram thecords of the invention react with a lower elongation than the knowncords, the load increase (C3-C2) being equal, that is the belt reallybehaves like a structure substantially inextensible in a circumferentialdirection, which ultimately brings about a better behavior of the tireof the invention, above all at high and very high speeds, as comparedwith the behavior of traditional tires.

Knowing this general behavior, it is possible to examine the process ofthe invention in more detail.

It is known that the process for the manufacture of tires usuallyprovides the following already mentioned and well-known steps:

manufacturing the tire carcass in the form of a cylindrical sleeve, byproducing a tubular element of rubberized fabric reinforced with cordsdisposed substantially parallel to the axis of said sleeve and foldingback the ends of the cords of said tubular element each about at leastan annular metal bead core which is circumferentially inextensible, saidbead cores lying in planes perpendicular to the sleeve axis;

toroidally shaping said tubular element by radially expanding the sleevewall centrally of the two bead cores and moving the bead cores close toeach other in an axial direction, so that a first toroidal configurationis achieved;

assembling a belt structure to said toroidally-shaped carcass, whichbelt structure comprises coils of cords oriented in a substantiallycircumferential direction and disposed in side by side relation, and atread band on said belt at a radially outer position thereof;

introducing the full carcass into a vulcanization mold of thecentripetal type;

closing said mold and expanding the carcass against the mold surface, soas to achieve the molding of the tread band in particular, and finallyvulcanizing said carcass by an appropriate heat treatment to hightemperature and pressure.

In accordance with the invention the above process is modified so as toaccomplish a new process characterized by the fact of:

making said belt using cords of the high-elongation type exhibiting, intheir load-elongation diagram, (see FIG. 2) a curvilinear portionmutually connecting two substantially rectilinear lengths of differentsloping, and

moving said annular bead cores coaxially in relation to each otherduring the toroidal shaping of said tubular element until a mutual axialdistance between the cores is reached which corresponds to anintermediate toroidal configuration of smaller diameter than that ofsaid first toroidal configuration and exhibits a deviation ofpredetermined value from said final toroidal conformation, the value ofthe center distance between said annular cokes being given by theload-elongation diagram of said belt cords, so that the expansion ofsaid carcass in said mold into the final toroidal configuration, putssaid belt cords in a tensile condition (preloading) corresponding to apoint of said diagram located within said curvilinear portion.

Shown schematically in FIG. 3 ape three different profiles (7, 8, 9) oftoroidal conformation of the selfsame carcass, profile 7 representingthe final conformation profile of the carcass plies in the vulcanizationmold after the molding has taken place and during the vulcanizationcycle. On all of the three profiles points A and B indicate the ends ofthe crown portion on which the belt structure is placed. It is obviousthat the profiles must have the same linear extension from one bead tothe other.

Profile 8 represents the first toroidal conformation profile of thecarcass, before applying the belt in accordance with the known art; thedifference D-d1 corresponds to the usual expansion at the equatorialplane undergone by the carcass and the belt during the pulling stepwhich, as seen, is equal to about 1.5%.

Profile 9 on the contrary represents the toroidal intermediateconformation profile of the selfsame carcass according to the process ofthe invention, that is that of the carcass ready to be assembled to thebelt formed with high-elongation cords.

The diameter of profile 9 has a value d2 lower than d1 so that theincrease D-d2 corresponding to the expansion of the carcass during thepulling step corresponds to the elongation undergone by the belt cords.

Said intermediate toroidal conformation profile 9 of the carcass can beadjusted so that it may have a lower diameter than the profile 8 byaxial movement of the beads to be spaced from each other at a convenientvalue c that is greater than b.

In this manner the expansion of the carcass as far as the profile 7 isachieved, is permitted by the fact that the mutual distance between thebeads, commonly known as "posting" simultaneously passes from value c tothe lower value a; in this manner it is in fact possible to achieve anincrease in the carcass diameter simultaneously with a constant linearextension of the toroidal profile of the carcass itself.

Ideally the difference D-d2 must correspond to the maximum depth s ofthe grooves in the tread pattern.

The elongation undergone by the belt cord or cords must be suitable toput said cords to a general point G (see FIG. 2) within the portion EFand preferably even to point r of the load-elongation diagram; in otherwords, G can vary at the inside of the knee but it will be preferablymoved as much as possible towards the knee outlet or even to theboundary thereof, that is at the beginning of the greatly slopingportion.

The importance of the characteristic proper to the process for themanufacture of tires in accordance with the invention is now readilyapparent and it consists in fixing the diameter of the toroidalconformation profile of the carcass depending upon the desired pullingvalue (in turn depending on the depth of the grooves in the treadpattern and/or the starting elongation of the cords used for the beltstructure) and to control said diameter value by the selection of acorresponding value c for the bead posting.

The correlation between the pulling amount and the groove depth is wellapparent; on the contrary as regards the high elongation cords it is tobe noted that the starting capability of elongation of said cords (firstdiagram length) depends on many factors, among which the stranding pitchof the wires and strands, so that different cords have differentcapability of starting elongation. Which means that, the desiredcondition for the cord at the end of the vulcanization step being fixedas that of point F and all other conditions being equal, the portion OFwill exhibit a variable length depending on the variation of the cordfeatures.

In other words, different starting elongations (OL) must be allocated tothe different types of cords so that they may reach the fixed conditionof point F and therefore also the pulling to be given to the carcassbefore its vulcanization must be of different amount.

It is to be noted that this pulling does not necessarily need occurwithin the mold but part of it could be occur on the conformation drumafter the belt has been assembled to the carcass.

It is instead important that the degree of pulling occurring within themold should not be lower than the maximum depth of the grooves in thetread pattern, which depth is also variable depending upon the type oftire and pattern.

It will be therefore clear how the use of said high-elongation cords inthe belt enables the desired value to be given to the belt pullingwhereas the manufacturing process in accordance with the inventionenables the pulling to be given to the belt, to be adjusted inaccordance with the precise features of the cord really used.

The manufacturing process being now known in its fundamental aspects,different embodiments are possible for the manufacture of the beltstructure on the toroidally conformed carcass, all of them fallingwithin the scope of the present invention.

According to a preferred embodiment the belt is made by helicallywrapping continuously and repeatedly around the circumference of thetire a single optionally rubberized cord into coils disposed in side byside relation, proceeding axially from one end to the other of the crownportion of said carcass.

The wrapping pitch can be greater than the cord diameter so as tosuitably control the density of the coils in the belt structure.

In this manner the angle at which the coils lie will not be strictlycircumferential with respect to the tire but the coils will be orientedaccording to angles v (FIG. 4) different from 0° relative to theequatorial plane.

However the value of said angle will always be lower than 2° and withinthis range, for high values of v, it could be convenient to modify alsothe lying angle of the cords 11 of the carcass plies (relative to thefundamental value of 90°) so as to maintain the correct cross anglebetween the carcass cords and the belt cords.

Preferably the coil density thus determined will be in the range of 25to 150 cords/dm.

Alternatively, instead of helically wrapping a continuous single cord, anarrow tape containing several cords but preferably no more than 5cords, could be conveniently wrapped around the carcass; in this mannerthe distance between the tape cords remains constant so that a possiblevariation in the tape coiling pitch p will bring about an unevendistribution of the coils in the belt structure thereby causing areaswith closer coils followed by areas with less dense coils.

FIG. 4 shows an example of this belt embodiment, obtained by helicallywrapping in an axial direction a tape 10 of rubberized fabric providedwith three HE cords 6 coiled according to a pitch corresponding to fivetimes the diameter of said cords.

In all cases of helical laying of the belt cords (either single cord oftape), the coiling can be conveniently carried out also starting fromthe middle of the crown portion, at the equatorial plane, and going onsimultaneously in an axial direction towards both ends of said crownportion, or the other way about.

ADVANTAGES OF THE INVENTION

Many advantages are achieved with the tire of the present invention.

First of all the fact that a pulling greater than the usual one can begiven to the raw carcass within the mold enables a perfect tire molding,in particular as far as the deepest grooves of the tread pattern areconcerned, whereas this capability of greater expansion of the belt onthe raw carcass is completely eliminated on the vulcanized tire andtherefore does not create disadvantages as regards the tire behavior inuse.

In addition to the foregoing, due to the fact that the tread molding nolonger takes place by penetration of the projections present on thesectors into the tread band thickness, it is not compulsory to use moldsof the centripetal type, and therefore simpler molds consisting of twohalves can be used.

In addition, the capability of greater expansion of the raw carcassimproves the quality level of the finished tire since it enablespossible unevenness in the structure and size of the used semi-finishedproducts to be compensated for, in particular as regards the tread bandthickness: it is in fact known that when the molding mainly takes placeby penetration of the projections disposed on the sectors into the treadband thickness even light deviations of the band thickness from thepredetermined values give rise to important irregularities in thedistribution of the elastomeric material and considerable deformationsin the underlying belt and carcass structure.

The process of the invention, by distributing this unevenness over thewhole circumferential and transverse extension of the carcass by effectof said capability of greater expansion, produces a levelling of theimperfections, a greater uniformity in the structure being recovered andtherefore accomplishing an increase in the quality level of the finishedtire.

As regards the belt, only the laying of the cord layer at 0° has beendescribed, but obviously it is understood that other cord layers orstrips can exist together with this layer, which layers may be inclinedto the equatorial plane and/or disposed circumferentially, according tothe known art.

The important advantages achieved with the tire of the invention, inparticular as regards the structural evenness and regular distributionof the rotating masses, are confirmed by the fact that the differencebetween the drift thrust values measured in the two rotationaldirections of the tire, is lower than 15% of the maximum detected thrustvalue.

It is obviously understood that the above description is to beconsidered as exemplary of, but not specifically limiting the presentinvention and therefore all modifications and variations that, thoughnot illustrated, can be easily deduced from the present inventive ideaby a person skilled in the art are to be considered within the scope ofthe invention itself.

I claim:
 1. In a process for manufacturing a radial tire for two-wheeledvehicles, said tire comprising a toroidal carcass of rubberized fabricplies with the plies lying in a toric final configuration and the endsof the cords of the plies folded back around metal bead cores, a treadband having a tread pattern on its surface comprising hollows orgrooves, and a belt structure between the carcass and the tread bandwhich includes substantially circumferentially oriented cords, said tirehaving a ratio of the tread radial height to the tread maximum widthbetween 0.15 and 0.45, said process comprising the steps of:(a)manufacturing a tire carcass in the form of a cylindrical sleeve ofrubberized fabric reinforced with cords substantially parallel to theaxis of said sleeve and folding back the ends of said sleeve each aboutat least one annular, circumferentially inextensible, metal bead core,said bead cores lying in planes perpendicular to the sleeve axis; (b)providing said belt structure with high-elongation, lang-lay type,rubberized metal cords having an ultimate elongation between 4% and 8%,and a load-elongation diagram having a curvilinear portion whose centerline ranges between an elongation value of 1.5% to 3% and which mutuallyconnects two substantially rectilinear lengths of different inclinationsrelative to the axes of said diagram, such that the cord is initiallymore elongated at low loads, (c) toroidally shaping said sleeve, byradially expanding the sleeve wall and moving said bead cores towardseach other in the axial direction to a first toric configuration oflesser diameter than that of said final configuration; (d) assemblingsaid belt structure to said toroidally shaped sleeve; (e) superimposingthe tread band on said belt structure at a radially outer positionthereof; (f) introducing the thus assembled tire into a vulcanizing moldand closing said mold; (g) expanding the tire radially outwardly againstthe mold surface, whereby the carcass plies reach said final toricconfiguration, the tread band is molded with said tread pattern and (h)vulcanizing said tire assembly, wherein the improvement comprises instep (c) above, shading said sleeve to a preselected toric configurationwhich is intermediate between the starting configuration of the sleeveand said first configuration, and selecting the diameter value of saidintermediate configuration in view of said load-elongation diagram ofsaid belt cords so that the subsequent expansion of said carcass in saidmold to the final toroidal configuration in step (g) places said beltcords in a tensile condition corresponding to a point on said diagramlocated within said curvilinear portion;. in step (d) above, assemblingsaid belt structure to said sleeve while it has said intermediateconfiguration.
 2. A process according to claim 1, in which said belt ismade by wrapping at least one strip of rubberized fabric reinforced withsaid high-elongation cords around said carcass, overlapping the ends ofsaid strips over a portion not lower than 20 mm in length.
 3. A processaccording to claim 1, in which said belt is made by wrapping a singlehigh-elongation cord continuously and repeatedly around said carcass andextending in an axial direction from one axial end of said carcass tothe other.
 4. A process according to claim 1, in which said belt is madeby helically wrapping a tape of rubberized fabric comprising two to tenhigh-elongation cords around said carcass, proceeding in an axialdirection from one end of the carcass to the other.
 5. A processaccording to claim 3, in which the helically wrapping of said metalhigh-elongation cords so as to make said belt is carried out bysimultaneously proceeding from the middle towards both axial ends of thecarcass.
 6. A process according to claim 4, in which the helicallywrapping of said metal high-elongation cords so as to make said belt iscarried out by simultaneously proceeding from the middle towards bothaxial ends of the carcass.
 7. A process for manufacturing a radial tirefor two-wheeled vehicles, said tire comprising a toroidal carcass ofrubberized fabric plies with the plies lying in a toric finalconfiguration and the ends of the cords of the plies folded back aroundmetal bead cores, a tread band having a tread pattern on its surfacecomprising hollows or grooves, and a belt structure between the carcassand the tread band which includes substantially circumferentiallyoriented cords, said cords being high-elongation lang-lay type,rubberized metal cords having an ultimate elongation between 4% and 8%,with a load-elongation diagram having a curvilinear portion whose centerline ranges between an elongation value of 1.5% to 3% and which mutuallyconnects two substantially rectilinear lengths of different inclinationsrelative to the axes of said diagram, such that the cord is initiallymore elongated at low loads, said tire having a ratio of tread radialheight to tread maximum width between 0.15 and 0:45, said processcomprising the steps of:manufacturing a tire carcass in the form of acylindrical sleeve of rubberized fabric reinforced with cordssubstantially parallel to the axis of said sleeve and folding back theends of said sleeve each about at least one annular, circumferentiallyinextensible, metal bead core, said bead cores lying in planesperpendicular to the sleeve axis; toroidally shaping said sleeve, byradially expanding the sleeve wall and moving said bead cores towardseach other in the axial direction to an intermediate toric configurationof lesser diameter than that of said final configuration, the differencebetween said diameters being not less than the depth of said grooves,and greater than 1.5% of the value of the lesser diameter elevating thediameter value of said intermediate configuration, depending upon theposition of said center-line in the load-elongation diagram so that thesubsequent expansion of said carcass in said mold to the final toroidalconfiguration places said belt cords in a tensile conditioncorresponding to a point on said diagram located within said curvilinearportion, by selecting a corresponding value for the moving of saidbeads, assembling said belt structure to said toroidally shaped sleeve,superimposing the tread band on said belt structure at a radially outerportion thereof, introducing the thus assembled tire into a vulcanizingmold and closing said mold, expanding the tire radially outwardlyagainst the mold surface, whereby the carcass plies reach said finaltoric configuration, the tread band is molded with said tread patternand said belt reinforcing cords are put in a tensile pre-loadedcondition corresponding to a point of their load-elongation diagramwithin said curvilinear portion; and vulcanizing said tire assembly.